MLB scorers have the benefit of replay. Most of us don't. If we get screened out by someone who stands up in front of us, lose the ball off the bat and are looking in the wrong direction when the play is not made or whatever, we don't get a second chance. We have to ask for help from others or just go with what we think we saw.
I have seen several calls by MLB scorers changed lately, usually an inning or two later. I don't know why it takes them that long, but maybe they review between innings and then it takes a while before the announcers calling the game catch on.
What do MLB scorers use as their tool? A book or a computer program? I would think its probably some kind of software but have never actually seen what they use.